A pregnant woman was stabbed in the chest in South Auckland after a love triangle turned bad last night.

The incident unfolded after a woman and two female friends drove to her ex-partner's house in Weymouth to assault his new girlfriend, a police spokeswoman said.

When the trio arrived about 10.30pm, they found several people drinking and a confrontation developed in the driveway.

After threats of violence the woman's friends fled the Taitimu Dr property and she sought refuge in a neighbour's house, where she asked for police be called.

But before they could respond, the woman was dragged out of the house and back to the original property.

More people then came out of the house and a violent altercation took place, the spokeswoman said, which allegedly involved the pregnant girlfriend of the woman's ex-partner.

The woman who planned the attack was knocked to the ground and was struck on the face and body as she lay there.

During the melee, the woman took out a small serrated steak knife she had concealed and lunged upwards, stabbing the pregnant woman in the chest. the spokeswoman said.

She then fled to another neighbouring property but was followed by associates of the pregnant woman.

A second altercation took place, which involved punching and hair-pulling, the spokeswoman said.

The woman eventually fled and hid before police arrived and found her.

The pregnant woman was taken to Middlemore Hospital and though initial reports suggested she had gone into labour, a Counties Manukau District Health Board spokeswoman said that was not the case and both mother and unborn child were in a stable condition.

Police spoke to witnesses last night and a scene examination was completed.

Photos of the scene would be taken today and other residents on the street would be interviewed.

A 20-year-old woman appeared in Manukau District Court today charged with assault.

Detective Inspector Mark Gutry said no decision had yet been made on whether the pregnant woman - also in her 20s - or any other person involved in the incident would face charges.